This invention relates to transmission devices. More particularly the invention relates to transmission devices of the type including a frame, a first element having a first axis fixed relative to the frame, and a second element rotatable about a second axis intersecting the first axis, the second axis being arranged for nutation about the first axis with the apex of the cone of nutation being the point of axes intersection. The first element has rolling surfaces of revolution disposed about the first axis and the second element has rolling surfaces of revolution disposed around the second axis. Provision is made for bringing the rolling surfaces of revolution around the axes into rolling engagement and drive input or output means is connected to at least one of the first and second elements.
In U.S. Pat. No. 3,955,432 issued May 11, 1976 to the present inventor, there is disclosed a transmission having a first element defining a pair of rolling surfaces of revolution about a first axis, a second element having a pair of rolling surfaces of revolution on a second axis intersecting the first axis and including a mechanical system for urging the rolling surfaces of the second element against those of the first element, which system is gyroscopic in origin. Specifically, gyroscopic means are associated with the second element to develop a gyroscopic couple which acts to retain the rolling surfaces of the second element against the rolling surfaces of the first element at two points located one on each side of a plane perpendicular to the first axis at the point of intersection thereof with the second axis. The gyroscopic forces thus developed are a combined function of the moment of inertia of the second element with respect to the second axis, the angle at which the first and second axes intersect, the rotational velocities of the second element around the second axis and nutational velocity of the second axis around the first axis. In this transmission, the gyroscopic force couple operates to both rock the second element around the point of axes intersection and maintain both rolling surfaces of the second element against both such surfaces of the first element in rolling friction contact.
To vary the ratio of input and output speeds of the transmission disclosed in this patent, provision is made to modify the angle of inclination of the second axis with respect to the first axis. As a result, the ratio of the radii of circles described by the points of rolling surface contact between the first and second elements, respectively, will be modified. Such a transmission is particularly well suited for the transmission of large forces due to the development of normal contact pressure by the gyroscopic forces while avoiding excessive axial forces on the transmission gear shafts as well as radial forces on the bearing supporting the second element.
Although the transmission disclosed in U.S. Pat. No. 3,955,432 possesses many desirable features, it is subject to certain drawbacks particularly when it necessary to vary the input and output speeds of the transmission over large ranges. This drawback is due primarily to the necessity for variation of the angle of inclination of the second axis with respect to the first axis. In other words, to obtain the rotary motion of the second element about its own axis as well as nutational movement of the second axis about the first axis, it is necessary to employ relatively complex mechanical coupling connections to the second element such as homokinetic joints, etc. In addition, the transmission of this patent is subject to drawbacks when the rotational velocity of the second axis about the first axis varies through transient ranges. During these transient ranges, the gyroscopic couple assumes values different from the nominal value in normal operation. The result is that the normal force creating the contact pressure deviates from its optimum value in such transient ranges. Although this drawback is of minor importance when the transmission operates continuously under normal conditions, it becomes a more serious problem when the transient ranges are more frequently incurred.
In U.S. Pat. Nos. 2,319,319, 2,405,957 and 2,535,409 transmission devices are described which comprise a plurality of second elements which are conical and convex in form and arranged in satellite manner about a first axis. The rolling surfaces of revolution formed on these conical elements are held in contact with an annular, toric, concave part at points of contact identical in number to the number of conical satellite elements. Each of these conical satellites is supported either at its two extremities by bearings located inside support plates having as their axis the first axis, as in U.S. Pat. No. 2,319,319, or at its small conical extremity by bearings located inside a support plate having as its axis the first axis, as in U.S. Pat. Nos. 2,405,957 and 2,535,409.
The pressure at the point of contact between the various conical satellites and the toric ring with which they cooperate is obtained by the combined effect of the centrifugal force acting on the conical satellites when the support plate or plates rotate and by a possible additional force applied radially to each of the satellites, such as springs, for example. In the transmission devices described in U.S. Pat. Nos. 2,319,319 and 2,405,957, the bearings are rotatably mounted inside the support plate so as to permit the satellite a certain degree of liberty in a radial plane. This arrangement allows the conical satellite to abut on the toric ring under the action of centrifugal force. In the transmission described in U.S. Pat. No. 2,535,409, the bearing in the support plate is blocked by means of a wedge system of such dimensions that it presses the conical satellites against the toric ring with a contact force which allows the transmission device to function in the absence of centrifugal force.
The concave toric ring is movable along its longitudinal axis, i.e. along the first axis, so as to modify the ratio of the radius R.sup.1 of the circles described by the points of contact on the reaction surfaces of the satellites to the constant radius R.sup.2 of the circles described by the points of contact of the reaction surface of the toric ring.
The rotation of the satellite support plate or plates draws each of these satellites in a movement around the surface of a cone with an apex angle a and having as its axis the first axis. Each of these satellites abuts on the rolling surface of the ring at a single point of contact. As the ring is immobile, when rotating about the axis of the ring (the first axis), the satellites react by starting to turn on themselves about their own axis (the second axis). The combination of the driving speed of the conical movement of each of these satellites and its rotational speed about its own axis is transmitted, by a planetary geartrain, to a drive transmission shaft coaxial with the first axis.
The transmission devices of these last mentioned U.S. patents have the following features:
(a) The force perpendicular to the rolling surfaces and exerted at the point of contact is minimum when the R.sup.1 :R.sup.2 ratio is high (point of contact at the large base of the cone, large contact surface), and is maximum when the R.sup.1 :R.sup.2 ratio is low (point of contact at the small base of the cone, small contact surface). This results in considerable variations in the specific contact pressure.
(b) The axial and radial reaction forces balancing out the contact pressure are accommodated by the bearing blocks supporting the conical satellites. These forces are directed in one direction or the other, depending on whether the points of contact are located on one side or the other of the satellite center of gravity. These forces are generally considerable, as they are of the same order of magnitude as the perpendicular contact force. The result is that the bearing blocks supporting the satellites must be designed and dimensioned so as to accommodate substantial forces during operation. Accordingly, it is difficult to exert very large forces at the point of contact.
(c) In addition to the centrifugal force which helps to keep the conical satellite lying in frictional rolling contact with the toric ring, a gyroscopic couple is also generated. In the case of these known transmission devices, this couple produces an undesirable effect, as its influence reduces the pressure at the point of contact and increases the radial force to be accommodated by the bearing blocks supporting the conical satellites.